Laser-guided munitions (generally referred to as laser-guided bombs (LGBs), laser guided weapon systems (such as in an aircraft), or laser-guided weapons (LGWs) use a laser designator to mark (illuminate, “paint”) a target. The reflected laser light (“sparkle”) from the target is then detected by the seeker head of the weapon, which sends signals to the weapon's control surfaces (fins) to guide it toward the designated point. The illuminating laser light is encoded, and the laser receiver in the LGW is set to react only to reflected laser light having the proper code.
An extensive discussion of laser designation techniques and procedures may be found in Joint Pub 3-09.1, Joint Tactics, Techniques, and Procedures for Laser Designation Operations, 28 May 1999, incorporated in its entirety by reference herein.
The earliest laser guidance seekers measured the intensity of the reflected laser light at four corners of the seeker window. (Normally, seekers use angle of incidence and when on axis, the focused spot hits the quadrant center; off axis, the spot moves to one quadrant.) The seeker then actuated the weapon's control fins to steer the weapon in the direction of the strongest signal return, thereby keeping the weapon centered on the laser sparkle. Later weapons have more sensitive seekers and more sophisticated control systems that waste less energy with course corrections, improving accuracy and range, but the principle remains essentially the same.
While LGWs are highly accurate under ideal conditions, they present a number of challenges to be used successfully, making them somewhat less than the “silver bullet” sometimes suggested. The first problem is designation. To insure accurate guidance, the target must ideally be illuminated for several seconds before launch, allowing the weapon's seeker to obtain a positive lock, and the target must remain illuminated during much of the weapon's transit time. If the designator's “sparkle” is turned off, blocked, or moved, the weapon's accuracy will be greatly reduced. Laser designation is also very vulnerable to weather conditions. Cloud cover, rain, and smoke frequently make reliable designation impossible. One patent describing a technique for attempting to validate the target is U.S. Pat. No. 5,350,134.
In the 1970s and 1980s it was common for aircraft to depend on a separate designator, either carried by ground forces, operated by the forward air controller, or carried by another aircraft in the strike group. It was often deemed more practical for one aircraft to provide lasing (perform designating) for its comrades. Modern conflicts and a growing emphasis on precision-guided weapons have pointed to the need for autonomous designation, and many fighter-bomber aircraft are now being fitted with designator pods to self-designate for laser-guided munitions.
One problem with LGWs is that there may be false returns from other than the desired target, such as from a nearby sand dune or vehicle. And, in some cases, mistakes are made and the wrong target can be attacked with potentially severe consequences.
It is known to have a spotter confirm that the correct target has been designated. SeeSPOT III, made by FLIR Systems Inc. (Wilsonville, Oreg. 97070 USA), is a hand held thermal and laser spot imager which uses reflected thermal energy from the laser to display like a video camera and display. It has very short range and because it essentially collects and integrates the light from the scene, it cannot decode the pulse timing. The laser spot is essentially seen as part of the scene and the spot has to compete with the background for visibility.
FIG. 1 illustrates an exemplary procedure for aircraft with laser-guided weapons (LGWs) and laser spot trackers, as set forth in Joint Pub 3-09.1, Joint Tactics, Techniques, and Procedures for Laser Designation Operations, 28 May 1999, incorporated in its entirety by reference herein, at page B-B-2.
In this scenario 100, an aircraft 102 is equipped with a LGW 104 which is shown already in its ballistic trajectory towards a target 106. Also illustrated is a forward air controller (FAC) 108, and a laser designator operator (LDO) 110. The FAC 108 is in radio communication with the pilot of the aircraft 102 and with the LDO 110. The LDO 110 illuminates the target 106 with a laser, and laser light is reflected back, typically as scattered reflections called “sparkle”.
Generally, the sequence of events is that the maneuver unit commander (not shown) decides to request close air support. The FAC coordinates laser code, laser target line and frequency and/or call sign of the LDO. The airstrike request includes laser-related data. An airstrike approval message is received, the FAC coordinates marking and air defense suppression. The aircraft is dispatched to a contact point to check in with FAC. The FAC coordinates laser code, laser-target line and frequency and/or call sign with LDO and pilot. Approaching the target, the aircraft calls in. The FAC relays laser control calls. The LDO designates the target (by illuminating it with laser beam). The aircraft acquires the target or releases the LGW. The LGW heads towards the target, adjusting its trajectory based on sparkle from the laser-illuminated target. The target is destroyed.
The situation, as described above, may vary, depending on the type of LGW used. Generally, LGWs home in on reflected laser energy to strike a target. Some LGWs require laser target illumination before launch or release and/or during the entire time of flight; some require illumination only during the terminal portion of flight. For example, designation delay can be used in HELLFIRE engagement when the missile is fired in a lock-on after launch (LOAL) mode. By delaying lasing (illumination of the target), the enemy has less time to react if they have laser warning receivers. In all LGW engagements, two-way communications greatly increase the chances of a successful engagement.
Laser designators can provide precision target marking for employment of air-to-surface and surface-to-surface LGWs. Precise target marking with laser designators is directly related to target size and aspect, laser-beam divergence, designation range, and atmospheric attenuation of the beam. Preferably, aircrews must always acquire targets visually. However, ground LDOs normally have more powerful optics to acquire targets, more time, and lower task loading than do aircrews of fighter or attack aircraft. The LDO may be either on the ground or airborne.
Laser illumination of a target requires an unobstructed line-of-of sight path between the laser designator and the target. In some cases, redundant laser designation is employed. This is a technique employing two or more laser designators in different locations but on the same code to designate a single target for a single LGW. For example, in the case of moving targets (such as a tank), using two designators may preclude a guidance failure as a result of temporary (line of sight) blockage (such as from intervening structures).
After illumination of the target, the aircrew must maneuver the aircraft to acquire the laser designator's energy using a laser spot tracker (LST). A visible mark may also be necessary to help the aircrew align the seeker.
Laser Codes
The aforementioned Joint Pub 3-09.1, Joint Tactics, Techniques, and Procedures for Laser Designation Operations, 28 May 1999, Chapter IV, incorporated in its entirety by reference herein, describes the laser codes. The following is extracted/edited therefrom.
Laser designators and seekers use a pulse coding system to ensure that a specific seeker and designator combination work in harmony. By setting the same code in both the designator and the seeker, the seeker will track only the energy with the correct coding. The seeker will track the first correctly coded, significant laser energy it sees. The seeker will always lock on to the most powerful return in its view. The pulse coding used by all systems discussed in this manual is based on pulse repetition frequency (PRF). (Laser codes are normally simple PRFs in the 10 to 20 Hertz range.)
The designator and seeker pulse codes use a modified octal system that uses the numerical digits “1” through “8.” The codes are directly correlated to a specific PRF (pulse repetition frequency), but the code itself is not the PRF and therefore can be communicated in the clear as required. Depending on the laser equipment, either a three- or four-digit code can be set. Three-digit code equipment settings range from 111 to 788. Four-digit code equipment settings range from 1111 to 1788. The three- and four-digit code equipment is compatible, and any mix of equipment can be used in all types of laser operations. However, when using a mix of three- and four-digit code equipment, all personnel must understand that the first digit of a four-digit code is always set to numerical digit “1”. The remaining three digits will be set to match the three digits of the three-digit code equipment. As an example, a three-digit code of 657 would be set to 1657 on a four-digit code system or vice versa.
The lower the code number, the faster the laser pulse rate. The lower code number and faster pulse rate will give the seeker the most opportunity to acquire the target in the time available, and is appropriate for the most important targets and the most difficult operating conditions. However, lower code numbers cause faster battery drain.
Coding allows simultaneous or nearly simultaneous attacks on multiple targets by a single aircraft, or flights of aircraft, employing LGWs set on different codes. This tactic may be employed when several high-priority targets need to be attacked expeditiously and can be designated simultaneously by the supported unit(s).
Certain codes (low code, high PRF, and/or faster pulse rate) are preferred for laser systems requiring precision guidance. Codes must be prebriefed to both the FAC and aircrews for situations where communications cannot be established or authorized.
Laser coding can be used effectively and securely with LGBs (LGWs). LGB codes are set on the bombs before takeoff and cannot be changed in the air. The aircrew is told the code, but advance coding information might not be sent to the supported ground unit. When the aircraft is on-station, the aircrew passes the code to the FAC. When the use of an LDO is required, the FAC coordinates with the LDO to ensure that the laser designator is set on the same code as the LGBs.
Laser Spot Trackers
A laser spot tracker is a sensor that picks up coded laser energy from a laser designator and projects a symbol on a sight or heads up display. Angle information may also be given to a weapons system. This symbol allows an operator to visually acquire the target designated by his or a friendly (LDO) laser. Most laser spot trackers are mounted on helicopters or fixed wing aircraft. It is believed that, at this time, there are no known ground-based systems with laser spot trackers, and it is believed that the only fixed wing aircraft with both a laser designator and a laser spot tracker are Navy F-18s and USAF fighters equipped with the Lightning II targeting pod, and European planes, Jaguar, Harrier and MRCA using the LRMTS (Laser Ranger and Marked Target Seeker) system. While the OH-58D, SH-60B, and HH-60H do not have laser spot trackers, pilots can see a laser spot if they are carrying a Hellfire Missile due to the missile seeker head cuing in their weapons display.
Some Patents of Interest
U.S. Pat. No. 5,350,134, incorporated in its entirety by reference herein, discloses target identification systems. A target identification system includes a target marker for selecting, and directing radiation at, a target, a weapon delivery system, and means for establishing a two-way communication channel between the two by reflection from a selected target. The communication is by infra-red laser and coded information is sent between the target marker and the weapon delivery system to identify the selected target.
U.S. Pat. No. 5,311,353, incorporated in its entirety by reference herein, discloses wide dynamic range optical receivers. A wide-dynamic range optical receiver amplifier is provided by using two separate amplifiers. The first amplifier is a low-impedance input, low-noise, high-gain amplifier, preferably a transresistance amplifier. An input resistor is chosen for the amplifier such that its resistance value is much greater than the input impedance of the first amplifier, resulting in insignificant change in input impedance when the first amplifier's output becomes saturated. A light-induced signal source is connected to the input resistor such that signal current from the light-induced signal source flows through the input resistor into the first amplifier input. A second high-input-impedance amplifier (preferably an FET-input buffer amp) is connected to receive the light induced signal source, either directly or through a resistive divider network. The difference in gain between the two amplifiers serves to extend the dynamic range of the optical receiver amplifier without switching input or feedback components, and without discontinuous response as the first amplifier becomes saturated. Other embodiments are directed to a further diode induced breakpoint, and to a front-end for a spot tracking system. As further disclosed therein,                Optical receivers (or opto-receivers) measure light used in various applications such as atmospheric studies, laser rangefinding, and spot tracking. In many applications it is desirable to utilize the value of light flux over a wide dynamic range. For example, a target-tracking (spot-tracking) device may provide directional information to a target by means of splitting a focused light spot reflected off of a target between four quadrants of a multi-sector photodetector. The distribution of light between the four quadrants of the photodetector provides an indication of how far “off-center” the detector is aimed. The light flux varies over a wide dynamic range as the tracker approaches the target, yet measurements must be taken.        FIG. 3a is a block diagram of a front end 300 for a spot tracking system utilizing wide dynamic range optical receivers of the type described hereinabove (e.g., 100, 200). In a spot-tracking system, a bright spot of light, e.g., a laser-generated spot, is focused on a target. Reflected light 305 from the target is imaged by an optical system 310 (e.g., a lens) onto a multi-sector photodetector 312 (described hereinbelow with respect to FIG. 3b). Signals from the multi-sector photodetector 312 are amplified by opto-receiver electronics 314 to provide a number (four shown) of sector (quadrant) output signals 316a, 316b, 316c, and 316d. In a spot tracking system, these signals are used to determine the position of the target relative to the orientation of the photodetector 312, and to provide guidance or steering signals to “aim” the photodetector 312 (or the device upon which it is mounted) more accurately towards the spot on the target.        FIG. 3b is a view of a four-quadrant multi-sector photodiode assembly 312 for use in a spot-tracking system such as that described with respect to FIG. 3a. The photodiode assembly 312 has an array of four distinct photosensitive areas (quadrants or sectors) 320a, 320b, 320c, and 320d. Each sector 320a, 320b, 320c, and 320d, has a first electrical connection 318a, 318b, 318c, and 318d, respectively. Second electrical connections to the each of the photo-sensitive areas are connected in common and provided as a single electrical connection 318e. The sensitive areas 320a, 320b, 320c, and 320d operate independently as separate photodiodes, each responding only to light impinging thereupon. Dashed line 305a indicates the effect of a perfectly centered spot illuminating each of the sensitive areas 320a, 320b, 320c, and 320d, equally.        It will readily be appreciated by one of ordinary skill in the art that although the four-quadrant photodiode assembly 312 shows four diodes connected in a common-cathode configuration, that multi-sector photo-diode assemblies can also be fabricated in a common-anode configuration, as discrete sector diodes, and with any number of sensitive areas. It is within the scope and spirit of the present invention to adapt any such single or multi-sector configuration of photodiodes for use with opto-receivers of the type described hereinabove with respect to FIGS. 1 and 2.        FIG. 3c is a schematic representation of the four quadrant photodiode 312 of FIG. 3b, wherein the four sensitive areas 320a, 320b, 320c, and 320d are represented graphically as four separate photodiodes with a common connection (318e).        FIG. 3d is a block diagram of a four-quadrant optical receiver system for use with a spot tracking system, according to the invention. (This figure corresponds roughly to the combination of 312 and 314 as described with respect to FIG. 3a.) In the figure, a four quadrant photodiode assembly is connected such that one sensitive area 320a, 320b, 320c, and 320d is connected to each of four opto-receivers 314a, 314b, 314c, and 314d, respectively. These opto-receivers 314a, 314b, 314c, and 314d are built according to the present inventive technique (e.g., 100—FIG. 1, or 200—FIG. 2).        While FIGS. 3a-d describe a “front-end” for a spot tracking system using a four-quadrant photodetector (the four quadrant photodiode 312), it will be readily appreciated by one of ordinary skill in the art that similar spot tracking systems are possible utilizing multi-sector detectors with two or more sensitive areas and a like number of opto-receiver amplifiers of the type described hereinabove. Where only linear tracking (one dimensional, e.g., up-down or left-right) is required, a “two-channel” system may be constructed using a two-sector photo-detector and two opto-receiver amplifiers (e.g., 100—FIG. 1 or 200—FIG. 2). A two dimensional (i.e., “X” and “Y”) spot tracking system can be constructed using a multi-sector photo-detector having three or more non-collinear sensitive areas and a like number of opto-receiver amplifiers.        It will also be readily appreciated by one of ordinary skill in the art that the photoconductive detectors described hereinabove (e.g., 101, 201, 312) may be provided by photodiodes (e.g. PIN diodes), phototransistors, or any other suitable photodetector device and that with an appropriate reversal of polarities, the principles of the present invention may be readily applied to negatively referenced or ground-referenced photodetectors. Accordingly, it should be recognized that the circuits described hereinabove are merely exemplary of physical configurations of this type and should not be considered as limiting the scope of the invention.        As further disclosed therein, the following patents generally disclose detecting and/or measuring light, especially laser light: U.S. Pat. Nos. 4,792,230 (measuring ultra-short optical pulses); 4,721,385 (FM-CW laser radar system); 4,830,486 (frequency modulated laser radar); 4,856,893 (which discloses both CW and pulse lasers, as well as range measurement); 4,812,035 and 4,846,571 (AM-FM laser radar).        
Commonly-owned U.S. Pat. No. 6,650,404, incorporated in its entirety by reference herein, discloses laser rangefinder receiver. In a laser rangefinder receiver, a return signal from a light-sensitive detector is passed through a high-pass filter, and is then processed in two separate circuit paths, a “signal” path and a “noise” path. The “signal” path employs a time-variable offset scheme to control receiver sensitivity. The “noise” path measures noise in the return signal, and maintain a noise-based threshold independent of the time-variable sensitivity of the “signal” path. No interstage coupling capacitors are employed, which contributes greatly to the receiver's quick saturation recovery. As further disclosed therein,                Laser rangefinders are well known, and are used to measure distances to targets. Generally, a laser transmitter is used to beam a high intensity pulse of light onto a selected target. The light scattered from (echoed or reflected off of) the target is detected by an optical receiver (or “opto-receiver”) which is normally located in close proximity to the laser transmitter. By measuring the transit time (time-of-flight) between a transmitted laser pulse and the received echo, the range (distance) to the target can be determined using a time-interval counter.        